1. Field of the Invention
This invention relates to a method and apparatus for illumination of electronic display devices, and more particularly to a high efficiency backlighting system suitable for back-illuminating transmissive electronic display devices.
2. Description of Related Art
Transmissive electronic display devices, such as active matrix liquid crystal displays (AMLCDs) or passive matrix liquid crystal displays (PMLCDs), are not emissive displays. That is, such transmissive displays are not themselves a source of light, as is, for example, a cathode ray tube (CRT). Accordingly, transmissive displays require a source of rear or back illumination to be usable in most applications and under widely varying ambient lighting conditions.
FIG. 1 shows a typical use of a flat panel transmissive display 1 as an output device (i.e., a visual display). (In the example shown, a graphical map 2 of a street intersection is displayed.) Typical uses of transmissive displays include sensor indicators and information outputs for automobiles, aircraft, scientific instruments, computers, airport information terminals, etc.
In the prior art, transmissive displays of the type shown in FIG. 1 have typically been backlit by bent or serpentine-shaped fluorescent lamp mounted behind the rear surface of the transmissive display. FIG. 2 diagrammatically shows a typical shape for such a fluorescent lamp 3. An essentially conventional straight fluorescent tube is heated and bent to the shape shown. Such a lamp 3 may be used in a reflective cavity and with a diffuser to provide a rudimentary backlight "system" behind a transmissive display. A fluorescent lamp backlight system is proven technology with moderately low development risk for most applications. Because of the low risk factor, moderate cost, efficiency, and limited alternative approaches, serpentine fluorescent backlighting systems are the primary backlighting approach for many military and commercial flat panel transmissive displays.
However, performance parameters and operating conditions of transmissive displays can vary greatly. For example, commercial transmissive displays do not require the same level of performance and operational ruggedness as military or automotive display devices. Consequently, backlight systems can vary in complexity, effectiveness, efficiency, and reliability, depending on the performance and operational requirements of their associated transmissive display. In general, prior art backlight systems do not have the characteristics necessary to be used in a wide range of applications. In particular, it has been found that conventional prior art fluorescent backlight systems have a number of problems, most notably:
Poor luminous efficiency PA1 Poor uniformity of lighting PA1 Short useful life PA1 Narrow dimming range PA1 High heat generation PA1 High ignition voltages PA1 External heating required for cold starts PA1 Improved luminous efficiency PA1 Improved uniformity of lighting PA1 Longer useful life PA1 A large dimming range (bright to dim ratio) PA1 Reduced heat generation PA1 Low ignition voltages PA1 No required external heating for cold starts
Accordingly, there is a need to provide a high efficiency backlighting system that is suitable for back-illuminating transmissive electronic display devices and which overcomes these problems. The present invention provides such a system.